GCSE · Biology · Edexcel · Spec 1BI0

Correction of eye defects

Two friends both say everything's blurry. One can read a menu but not the departures board; the other, the exact reverse. Same complaint, opposite problem — and opposite fixes.

Diagnose, then correct

Where do the light rays meet?

Pick where the problem is, then follow the route to the defect and its fix. Try all three.

Where it goes wrong → The defect and its cause → The fix

3 routes to a fix.

On Vision is blurry. 3 branches to choose from.

Each problem takes exactly one route, and each route ends in its own fix.

Watch out: Short- and long-sightedness are both about WHERE the rays meet. A cataract is a different kind of problem: the lens itself has gone cloudy.

Where the rays meet

Put each focal point in its place

This line is a picture of the idea, not a measurement. 0 is the retina. Negative numbers are in front of the retina (towards the lens); positive numbers are behind it. Each faulty eye misses by one step — your job is to decide which way. Place all four focal points.

The fix that feels backwards

?

Reason it through

Why does a lens that spreads light out fix short-sightedness?

Link 1 of 4

First link · your turn

Tempting guess: the eye is 'too weak', so give it a stronger lens. Before you trust that, where do the rays actually meet in a short-sighted eye?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Ways to correct vision

What does each option actually do?

Tick every property each option has. Then check your grid against the real one — watch the laser eye surgery row.

Spectacles
Contact lenses
Laser eye surgery
Lens replacement (for a focusing defect)
Cataract surgery

Now say it yourself

Explain the convex fix

Jo is long-sighted. Explain how wearing convex lenses lets Jo see near objects clearly. [4 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Find where the light rays meet — then move them onto the retina.

What you need to know

  • Blurry vision from short- or long-sightedness means the focal point is missing the retina.
  • Every correction for those two defects does one job: it moves the focal point onto the retina.
  • A cataract is a different problem — the lens itself goes cloudy — and surgery replaces that lens.

The big picture

Short- and long-sightedness blur vision because the eye's lens can't focus light onto the retina: the focal point misses it. A concave lens moves the focal point back onto the retina for a short-sighted eye; a convex lens brings it forward for a long-sighted eye. Laser eye surgery reshapes the cornea instead, and cataracts — a cloudy lens — are treated by replacing the lens with an artificial one.

Key points

1Short-sightedness (myopia): the eyeball is too long, or the lens is too thick and curved and refracts light too strongly. The focal point falls short of the retina, so distant objects look blurry.
2A concave lens makes the rays diverge, which extends the focal length and moves the focal point back onto the retina.
3Long-sightedness (hyperopia): the lens has stiffened and can't become round enough (more common with age), or the eyeball is too short. The focal point falls beyond the retina, so near objects look blurry.
4A convex lens makes the rays converge, which shortens the focal length and brings the focal point forward onto the retina.
5Corrective lenses can be worn as spectacles or as hard or soft contact lenses. Laser eye surgery changes the shape of the cornea, not the lens.
6Replacing the lens with an artificial one is a bigger operation with its own risks. For a cataract, surgery removes the cloudy lens and puts in an artificial (plastic) one.

Worked example

Problem

Amir can read the small print on his phone easily, but the number on the front of a bus is blurry until the bus is close. Which defect does he have, where does the focal point fall, and which lens corrects it?

⚠ Watch out

Thinking a short-sighted eye is 'too weak' and needs a lens that focuses harder. It's the other way round: its focal point is already in front of the retina, so a concave lens has to spread the rays out to move the focal point back.

🧠

Memory hook

Short-sighted: the rays fall short. Long-sighted: they go too long. Whichever way they miss, push the focal point the other way — concave spreads the rays to push it back, convex gathers them to pull it forward.

✓

Check yourself

Cover the page. Long-sighted eye: where does the focal point fall, which lens fixes it, and what happens to the focal length? And what does laser eye surgery change?

Flashcards

(14)
What do short-sightedness and long-sightedness have in common?
The eye's lens can't focus light onto the retina. The focal point misses the retina, so the image is blurry.
Short-sighted eye: where does the focal point fall?
Short of the retina — in front of it.
Two causes of short-sightedness
An eyeball that is too long, or a lens that is too thick and curved, so it refracts light too strongly.
Short-sighted: which objects look clear, and which look blurry?
Near objects are clear; distant objects are blurry.
How does a concave lens correct short-sightedness?
It makes the light rays diverge, which extends the focal length so the focal point lands on the retina.
Long-sighted eye: where does the focal point fall?
Beyond the retina — behind it.
Two causes of long-sightedness
A stiffened lens that can't become round enough to focus light from close objects (more common with age), or an eyeball that is too short.
How does a convex lens correct long-sightedness?
It makes the light rays converge, which shortens the focal length and brings the focal point forward onto the retina.
In a ray diagram, how can you tell short-sightedness from long-sightedness?
Short-sighted: the rays cross before they reach the retina. Long-sighted: the rays converge to a point beyond the retina.
Two ways corrective lenses can be worn
As spectacles, or as hard or soft contact lenses worn instead of spectacles, which are much less visible on the face.
Laser eye surgery changes which part of the eye?
The cornea — its shape — not the lens. That changes how much the cornea refracts light, so light is focused on the retina.
Why is replacing the lens not a common fix for short- or long-sightedness?
It is a more substantial surgical procedure, and it carries its own risks.
What is a cataract?
The lens becomes cloudy (opaque). Vision becomes blurry, colours look faded and bright light causes glare.
How are cataracts treated?
Surgery removes the cloudy lens and replaces it with an artificial (plastic) lens.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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